Electrostatic discharge (ESD) is one of destructive and unavoidable phenomena that electric systems and integrated circuits are attacked. From the electric view, ESD is a transitional high current phenomenon such that a peak current of several amperes continues for a period of time from 10 n sec to 300 n sec. Therefore, when ESD is generated, an integrated circuit is damaged so that it is hardly repaired, or its volume breaks down or deteriorates so that it does not work normally unless a current of several amperes is conducted outside the integrated circuit within several nano sec. Furthermore, the lightening, thinning and downsizing for electronic parts or electronic apparatuses advance rapidly in recent years. With the rapid progress, the degree of integration in semiconductors and the density of electronic parts mounted on printed wiring boards are increased remarkably with the result that integration is minute, or electronic elements or signal conductors mounted are present very closely each other. Under these circumstances, high frequency radiation noise is easily caused together with the accelerating of the signal-processing rate.
Conventionally, as an element for protecting from static electricity, which protects IC etc. in circuits from ESD, JP-A-2005-353845 discloses an element having a bulk structure, which comprises a sintered matter of a metal oxide etc. This element is a laminated chip varistor made from the sintered matter and is equipped with a laminate and one pair of external electrodes. The varistor has a property such that when the applied voltage reaches a certain value, the current, which has not been flown until now, begins to flow suddenly, and also has excellent blocking force to electrostatic discharge. However, in the production of the laminated chip varistor, which is a sintered matter, a complicated production process including sheet molding, internal electrode printing, sheet lamination, etc. is inevitable and the production also has a problem that delamination and other wrong conditions are easily caused during mounting.
As an electrostatic discharge element for protecting IC and the like in a circuit from ESD, there is a discharge type element. The discharge type element has merits that a leak current is small, the principle thereof is simple and breakdown is hardly caused. Furthermore, in the discharge type element, the discharge voltage can be regulated by the distance of a discharge gap. When the element has a sealing structure, the discharge voltage can be determined by the gas pressure or the gas kind. As a commercially available element, there is an element prepared by forming a cylindrical ceramic surface conductive coating film, providing a discharge gap on the coating film by means of leaser and glass sealing the gap. This glass sealing tube type discharge gap element has excellent properties, but has a limitation on downsizing as a surface-mounting element because of having its complicated form and also has a difficulty in decreasing the cost.
The method of forming a discharge gap on a wiring directly, and regulating the discharge voltage by the distance of the discharge gap is disclosed in the following documents. For example, JP-A-H3 (1991)-89588 discloses that the distance of the discharge gap is 4 mm, and JP-A-H5 (1993)-67851 discloses that the distance of the discharge gap is 0.15 mm. In the smallest gaps according to the conventional techniques, the discharge voltage between the parallel electrodes is about 1 kV or higher. In the protrudent electrodes, the discharge voltage is decreased by about 10 to 20 percents, but this discharge voltage is too high for protecting IC or LSI having a low power supply voltage.
Moreover, when the discharge gap part is not protected, it is presumed that the surface of a conductor is contaminated by humidity or gases in the environment and the discharge voltage is changed. For protecting the discharge gap, it is unpractical to fill the discharge gap with a conventional resist directly because the discharge voltage is vastly increased. When the narrow discharge gap having a distance of about 1 to 2 μm or smaller is filled with a conventional resist, the discharge voltage can be decreased, but the resist filled suffers slight deterioration to cause problems such that the insulating resistance is lowered or continuity occasionally occurs.
The distance of a discharge gap and the optimization of the discharge voltage are disclosed in JP-A-H10 (1998)-27668. It discloses that the discharge gap is preferably 5 to 60 μm for protecting general electronic elements, and the discharge gap is preferably 1 to 30 μm for protecting IC or LSI, which is more sensitive to static electricity and particularly, the discharge gap can be increased to about 150 μm for the use that only a large pulse voltage portion is removed.
With respect to a relationship of the product of the distance of a discharge gap and pressure with discharge voltage, there is a Paschen's law. In the Paschen's law, when the distance of a discharge gap is 7 μmin the air (1 bar, 20° C.), the discharge voltage is determined to 350 V.
As described above, when a discharge gap having a distance of several μm to 50 μm is filled with an insulating resin, the discharge voltage correlating to the distance can be obtained in principle. However, when the application of 8 kV is carried out to human body charged model (HBM) according to IEC61340-3-1 in practical, the insulating resistance is lowered according to the kind of a resin or continuity occurs to cause a problem that it is not endurable to practical use.
Specifically, the resin filled in a discharge gap is preferably a material having high flame resistance and low deterioration by discharging. Examples of such heat resistant resin are fluorine resins such as polytetrafluoroethylene, tetrafluoroethylene/hexafluoropropylene copolymer and tetrafluoroethylene/perfluoroalkoxyethylene copolymer; epoxy compounds such as polyimide resin, alicyclic epoxy resin and novolak type epoxy resin; polyquinoxaline, polyquinoline, polybenzoimidazole, polybenzothiazole, polybenzoxazole, benzimidazobenzo phenanthroline type ladder polymer, poly-4-hydroxy benzoate, silicon resin etc. In a wiring board that a flexible substrate comprises polyimide as a base and the discharge gap between electrodes is 15 μm when to the discharge gap obtainable by filling with a heat resistant resin and curing, a pulse voltage of 8 kV in HBM model is applied, the insulating properties are changed and the electric resistance value is apt to be lowered. Namely, it is turn out that when the insulating resin for protecting a discharge gap merely has heat resistance, the polyimide as a base suffers deterioration to induce short circuit. Furthermore, it is turn out that the resin has limitation on a solvent used for dissolution and has some other problems for practical use thereof.